The present invention relates to a device for the internal machining of a tube. It particularly applies to the machining of the inner bead of a weld between two long tubes.
For certain applications, such as heat exchangers, is it sometimes necessary to have very long tubes, which are normally formed by the butt welding of several smaller tubes. For example for coils, these tubes are subject to pressures of several hundred bars and to temperatures of several hundred degrees celsius.
The present welding processes (TIG, electron beam, etc) have difficulty in preventing the formation of a welding bead within the tube at the point where the two welded tubes are joined.
However, in view of the aforementioned operating conditions, such an internal bead leads to stress concentration at such junction, which is prejudicial to the satisfactory fatigue behaviour of the tube. Moreover, during the inspection of welds by means of probes, the internal bead can prevent the introduction of such probes, or lead to the wear thereof by friction, or can distrub the continuous advance of the supply cables for such probes by inducing spurious signals in them. It is for this reason that it is very advantageous to machine the internal bead of the weld between two tubes.
Hitherto, two known methods are conventionally used for machining said bead.
A first method consists of mounting a tool at the end of an extension piece which is rotated during machining. This method has the major disadvantage of being unusable in the case of very long tubes, the extension piece being unable to withstand the effective twisting torque during machining. The extension piece which is subject to twisting can then be the location of torsion vibrations which are incompatible with correct machining.
A second method consists of forcing scrapers through the tube in one pass or giving them a reciprocating movement, the pass or passes ensuring the machining of the bead. The major disadvantage of this method is that the scrapers scratch the interior of the tube during their displacements. The thus-produced scratches can give rise to disturbing stress concentrations and in particular can assist the corrosion of the tube interior.